The present invention relates to a process for producing bis-xcex2-hydroxyethyl terephthalate and/or a low condensate thereof from an aromatic polyester and to a process for purifying bis-xcex2-hydroxyethyl terephthalate or a low condensate thereof. More specifically, it relates to a process capable of producing bis-xcex2-hydroxyethyl terephthalate and/or a low condensate thereof efficiently even from an recovered aromatic polyester and to a process capable of purifying bis-xcex2-hydroxyethyl terephthalate and/or a low condensate thereof obtained by the above process, to a high level.
One of the characteristic features of aromatic polyesters is that they have excellent performance suitable for use in the field of a wide variety of molded products such as fibers, films or resins. Another characteristic feature of the aromatic polyesters is that it is relatively easy to return them to a raw material stage by depolymerization.
Aromatic polyesters, especially terephthalate-based polyesters centering on polyethylene terephthalate are widely used in the field of various molded products as described above. As means of producing an aromatic polyester, there is currently used a process comprising the steps of forming an intermediate containing bis-xcex2-hydroxyethyl terephthalate by a direct esterification reaction between terephthalic acid and ethylene glycol or an ester exchange reaction between a lower alkyl ester of terephthalic acid, especially dimethyl terephthalate, and ethylene glycol and then, generally subjecting the intermediate directly to the polycondensation step at a high temperature under high vacuum. The aromatic polyester can be returned to a raw material stage by depolymerization and polycondensed again to produce a polyester. Therefore, it can be said that it is an excellent material from the viewpoint of resource saving because it can be recycled.
To depolymerize a terephthalate-based polyester, a depolymerization reaction has been heretofore generally carried out basically in a reaction system containing the polyester and an excess of ethylene glycol as the main constituent components. In order to carry out the reaction smoothly, it is important to create a state where the polyester is easily depolymerized. When an amorphous polyester is used as a raw material, the depolymerization reaction proceeds relatively easily. However, when the polyester is in the form of a fiber having high crystallinity, the depolymerization reaction relatively hardly proceeds in most cases. When the polyester is in the form of a bottle which is a resin molded product, there is a tendency that depolymerization hardly proceeds in a neck portion where crystallization is in progress, of the bottle.
On the other hand, the purification of crude bis-xcex2-hydroxyethyl terephthalate, obtained by depolymerization, to a high level is very important when a high-quality aromatic polyester is to be produced using the bis-xcex2-hydroxyethyl terephthalate again as a raw material.
A typical method for the purification of the crude bis-xcex2-hydroxyethyl terephthalate is a purification method by recrystallization. Although bis-xcex2-hydroxyethyl terephthalate obtained by this method appears to have high quality, impurities still remain practically and become obstacles to the acquisition of a high-quality polyester in most cases. Particularly when a polyester is to be recovered and depolymerized into bis-xcex2-hydroxyethyl terephthalate to obtain a polyester again, the above obstacles are markedly observed in most cases.
Meanwhile, JP-A 48-86841 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a process for purifying bis-xcex2-hydroxyethyl terephthalate or a mixture of bis-xcex2-hydroxyethyl terephthalate and terephthalic acid by treating it with water and then separating water, wherein the separated water is treated with an ion exchange resin and absorbent, and recycled for re-use.
In the above purification process, bis-xcex2-hydroxyethyl terephthalate is treated with water for purification and an ion exchange resin or absorbent is not used directly for the purification of bis-xcex2-hydroxyethyl terephthalate. Disclosure of the Invention
It is an object of the present invention to provide a process for producing bis-xcex2-hydroxyethyl terephthalate and/or a low condensate thereof from an aromatic polyester.
It is another object of the present invention to provide a process for producing bis-xcex2-hydroxyethyl terephthalate and/or a low condensate thereof, which can efficiently decompose a highly crystallized aromatic polyester, for example, even a recovered aromatic polyester fiber, fiber layer or film, or a recovered bottle having a highly crystallized neck portion.
It is still another object of the present invention to provide a process for producing high-purity bis-xcex2-hydroxyethyl terephthalate.
It is a further object of the present invention to provide a process for producing high-purity bis-xcex2-hydroxyethyl terephthalate by removing, as much as possible, ions which are impurities contained therein inevitably, for example, ions derived from an esterification catalyst or an ester exchange catalyst.
It is a still further object of the present invention to provide a process for producing high-purity bis-xcex2-hydroxyethyl terephthalate from bis-xcex2-hydroxyethyl terephthalate obtained by glycol-decomposition of a recovered polyethylene terephthalate.
It is a still further object of the present invention to provide highly purified bis-xcex2-hydroxyethyl terephthalate having an extremely small content of cations and/or anions.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a process for producing bis-xcex2-hydroxyethyl terephthalate and/or a low condensate thereof from an aromatic polyester (may be referred to as xe2x80x9cfirst process of the present inventionxe2x80x9d hereinafter), comprising the steps of:
heating an aromatic polyester comprising terephthalic acid as a main dicarboxylic acid component and ethylene glycol as a main glycol component together with bis-xcex2-hydroxyethyl terephthalate and/or a low condensate thereof to pre-decompose the aromatic polyester; and then,
reacting the obtained pre-decomposed product with ethylene glycol to convert the terephthalic acid component of the pre-decomposed product into bis-xcex2-hydroxyethyl terephthalate and/or a low condensate thereof.
According to the present invention, secondly, the above objects and advantages of the present invention are attained by a process for producing bis-xcex2-hydroxyethyl terephthalate having a small content of ions (may be referred to as xe2x80x9csecond process of the present inventionxe2x80x9d hereinafter), comprising the step of:
bringing a bis-xcex2-hydroxyethyl terephthalate solution composition comprising ethylene glycol, bis-xcex2-hydroxyethyl terephthalate and cations and/or anions as impurities into contact with a cation exchanger and/or an anion exchanger to reduce the total content of cations and anions as impurities to 50 ppm or less based on the bis-xcex2-hydroxyethyl terephthalate.
According to the present invention, thirdly, the above objects and advantages of the present invention are attained by a process for purifying bis-xcex2-hydroxyethyl terephthalate (may be referred to as xe2x80x9cthird process of the present inventionxe2x80x9d hereinafter), comprising the steps of:
(1) obtaining a crude bis-xcex2-hydroxyethyl terephthalate by subjecting a bis-xcex2-hydroxyethyl terephthalate-containing mixture containing cations and anions in a total amount of 50 ppm or less, ethylene glycol in an amount of more than 10 wt % and a compound having a boiling point lower than that of bis-xcex2-hydroxyethyl terephthalate to preliminary evaporation or distillation to distill off the compound having a boiling point lower than that of bis-xcex2-hydroxyethyl terephthalate; and then,
(2) subjecting the crude bis-xcex2-hydroxyethyl terephthalate to evaporation or distillation under reduced pressure to separate purified bis-xcex2-hydroxyethyl terephthalate.
Further, according to the present invention, fourthly, the above objects and advantages of the present invention are attained by a process for purifying bis-xcex2-hydroxyethyl terephthalate (may be referred to as xe2x80x9cfourth process of the present inventionxe2x80x9d hereinafter) comprising the step of:
subjecting crude bis-xcex2-hydroxyethyl terephthalate having a cation and anion total content of 50 ppm or less to evaporation or distillation under reduced pressure.
Finally, according to the present invention, the above objects and advantages of the present invention are attained by purified bis-xcex2-hydroxyethyl terephthalate containing cations and anions in a total content of 15 ppm or less. Preferable Embodiment of Invention